Thermal Energy Storage Systems Utilizing Phase Change Materials

ABSTRACT

A thermal energy storage system utilizes a phase change material with an encapsulating material surrounding the phase change material. The encapsulating material fully contains the phase change material to prevent passage of the phase change material out of the encapsulating material and/or prevent direct contact of the phase change material with external objects. The encapsulating material is a high density polyethylene (HDPE), which may be a non-crosslinked or crosslinked HDPE. In a method of forming a thermal energy storage system a phase change material is surrounded with the encapsulating material.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Patent Application No. 62/567,534, filed Oct. 3, 2017, which is incorporated herein by reference for all purposes.

TECHNICAL FIELD

The invention relates to thermal energy storage systems utilizing phase change materials, and their manufacture and use.

BACKGROUND

Phase change materials (PCMs) used in thermal energy storage (TES) systems are materials that can store a large amount of latent heat. The PCMs work due to their phase change transition temperatures, typically their melting or freezing points where the material transitions between solid and liquid phases. When a PCM is cooled to a temperature at or below its melting point temperature it forms a solid. When this material is placed in an area that is above the PCM's melting point, the PCM will absorb the heat from the surrounding area. If the PCM is at a temperature below its melting point temperature, the temperature of the PCM will begin to rise as a result of absorbing the surrounding heat. The heat absorbed during this stage is referred to as sensible heat wherein the temperature of the PCM rises as it absorbs heat. When the temperature of the PCM rises to its melting point temperature as a result of heat absorption, the temperature of the PCM will stop rising. Instead, the temperature of the PCM will stay at the melting point temperature as heat is continued to be absorbed from the surrounding area as the material transitions to its liquid phase. The heat absorbed during this stage is referred to as latent heat. Because the heat absorbed is latent heat, the heat is not reflected in a rise in temperature but in a transition of the PCM from a solid to a liquid. When all of the PCM is melted so that all of the PCM is in a liquid state, only then does the temperature of the PCM begin to rise again due to the absorption of heat.

PCM materials can be selected so that they can absorb a large amount of latent heat and thus will typically stay at the melting or freezing point temperature for long periods as the PCM continues to absorb latent heat. By selecting PCMs with desired melting points or phase transition temperatures, the PCM can be placed in an environment and used to keep the environment at or near a desired temperature for long periods of time. This is beneficial in that PCM materials can be used to replace or supplement cooling or powered refrigeration equipment, such as used in storage, shipping, etc., where materials, such as food, medical supplies, electronics, bio-samples, etc., must be maintained in cool or chilled environment. In certain instances, refrigeration equipment to cool the PCM materials to desired temperatures can use off-peak electricity so that during peak electricity use, the PCM material can be used in areas where cooling or refrigeration is needed without using electrical equipment and thereby reducing electricity requirements during the peak usage times.

One of the limitations of PCM materials is the ability to keep them contained. Because the PCM materials transition between solid and liquid states, the liquid PCM must be prevented from leaking or spilling, damaging items or affecting the surrounding environment. In certain instances, the PCM may be corrosive or hazardous in nature so that containment is essential. Some PCMs may be flammable or cause odor issues if not contained. Furthermore, the PCM may degrade or become contaminated over time upon exposure to atmospheric conditions if not contained, reducing their effectiveness.

The container, shell or encapsulating material used to contain the PCMs must have sufficient mechanical strength that is maintained under the conditions in which the PCM is used. This includes maintaining its integrity under the temperatures and packing loads encountered. Further, the container or encapsulating material must be compatible with the PCM. While various containers and encapsulating materials have been used to contain PCMs, improvements are needed.

SUMMARY

A thermal energy storage system includes a PCM and an encapsulating material surrounding the PCM. The PCM is fully contained therein to prevent passage of the PCM out of the encapsulating material and/or prevent direct contact of the PCM with external objects. The encapsulating material is a high density polyethylene (HDPE).

In particular embodiments, the HDPE encapsulating material has an environmental stress crack resistance (ESCR) while in contact with phase change materials at 5° C. to 27° C. of from 100 hours or more. The PCM may have a liquid-solid phase transition temperature of from 50° C. or less at standard pressure. In certain instances, the PCM may have a liquid-solid phase transition temperature of from 10° C. or less at standard pressure.

In specific applications, the PCM may be selected from at least one of an alkane, an alcohol, an organic acid, an ester, a polyethylene glycol, an inorganic salt hydrate, a mixture of inorganic salts and/or inorganic hydrates, eutectic mixtures of organic-organic materials, eutectic mixtures of organic-inorganic materials, and eutectic mixtures of inorganic-inorganic materials.

In certain embodiments the HDPE material of the encapsulating material is a cross-linked HDPE. In such instances, the material may have from 90% or less crosslinking. The encapsulating material may have a wall thickness of from 3 mm or less. In some cases, the encapsulating material may contain a heat conducting additive.

In particular applications, the thermal storage system is formed in at least one of 1) a sphere having a diameter of from 10 mm to 40 mm, and 2) an elongated cylindrical body having a diameter of from 2 to 5 mm.

In a method of forming a thermal energy storage system, a PCM is surrounded with an encapsulating material so that the PCM is fully contained therein to prevent passage of the PCM out of the encapsulating material and/or to prevent direct contact of the PCM with external objects. The encapsulating material is a high density polyethylene (HDPE).

In particular embodiments of the method, the HDPE encapsulating material has an environmental stress crack resistance (ESCR) while in contact with PCMs at from 5° C. to 27° C. of from 100 hours or more. The PCM may have a liquid-solid phase transition temperature of from 50° C. or less at standard pressure. In certain instances, the PCM may have a liquid-solid phase transition temperature of from 10° C. or less at standard pressure.

In specific applications, the PCM may be selected from at least one of an alkane, an alcohol, an organic acid, an ester, a polyethylene glycol, an inorganic salt hydrate, a mixture of inorganic salts and/or inorganic hydrates, eutectic mixtures of organic-organic materials, eutectic mixtures of organic-inorganic materials, and eutectic mixtures of inorganic-inorganic materials.

In certain embodiments the HDPE material of the encapsulating material is a cross-linked HDPE. In such instances, the material may have from 90% or less crosslinking. The encapsulating material may have a wall thickness of from 3 mm or less. In some cases, the encapsulating material may contain a heat conducting additive.

In particular applications, the thermal storage system is formed in at least one of 1) a sphere having a diameter of from 10 mm to 40 mm, and 2) an elongated cylindrical body having a diameter of from 2 to 5 mm.

In some embodiments, the encapsulating material is formed into a selected shape by at least one of injection molding, gas-assisted injection molding, water-assisted injection molding, thermo-forming, blow molding, blown film extrusion, and 3D-printing.

DETAILED DESCRIPTION

Phase change materials (PCMs) for use in thermal energy storage (TES) systems can supply heating or cooling at desired temperatures. Because of their unique characteristics, PCMs can be used in a variety of applications. These may include such things as stabilizing or maintaining the indoor temperature of a room or building, the temperature of containers or boxes, such as those used for shipping or transport, the temperature of food or drinks, the temperature of electronic devices and batteries, the temperature of medicine and medical supplies, the temperature of biological material or samples, etc.

As discussed earlier, the PCM works due its ability to absorb large amounts of heat. This is typically through the absorption of latent heat wherein the PCM absorbs heat at its melting or freezing point, wherein the material transitions between its liquid and solid phases. By selecting and utilizing PCMs with a particular liquid-solid phase transition temperature (i.e., the melting or freezing point) the PCM can be used to keep a surrounding environment at or near this temperature. Unless stated otherwise, all melting/freezing points or phase transition temperatures presented herein are those at standard or atmospheric pressure.

The PCMs may be either organic or inorganic and may include clathrates. Organic PCMs can include alkanes or paraffins, fatty alcohols, fatty acids, fatty esters, polyethylene glycols, etc. Sugar alcohols, which may have melting temperatures in the range of from 90° C. to 120° C., may also be used in certain applications. Non-limiting examples of organic PCM materials with a melting temperature in the range of from 1° C.-20° C. include n-tetradecane (MP=4-6° C.), n-pentadecane (MP=8-10° C.), n-hexadecane (MP=18° C.), n-heptadecane (MP=19° C.), n-decanol (MP=5-7° C.), nonanoic acid (MP=12.5° C.), octanoic acid (MP=16.7° C.), methyl laurate (MP=4-5° C.), methyl myristate (MP=18° C.), polyethylene glycol-400 (MP=8° C.), and polyethylene glycol-600 (MP=20° C.),

Inorganic PCMs can include inorganic salts or inorganic salt hydrates or mixtures of inorganic salts and/or inorganic salt hydrates. Non-limiting examples of inorganic PCM materials with a melting temperature in the range of 1° C.-20° C. include lithium chlorate trihydrate (LiClO₃·3H₂O) (MP=8° C.), and potassium fluoride trihydrate (KF·3H₂O) (MP=18.5° C.).

Non-limiting examples of clathrates include tetrahydrofuran (THF)+H₂O (MP=5° C.) and tetrabutylammonium bromide+H₂O (MP=12° C.).

Eutectic mixtures of compounds having desired melting points may also be used as the PCMs. Eutectic mixtures may include eutectic mixtures of organic-organic materials, eutectic mixtures of organic-inorganic materials, and eutectic mixtures of inorganic-inorganic materials. Non-limiting examples of PCM materials consisting of a mixture of inorganic salts and/or their hydrates with a melting temperature in the range of 1° C.-20° C. include a mixture of ammonium chloride, sodium acetate and sodium formate (MP=5° C.), a mixture of disodium hydrogenphosphate dodecahydrate & dipotassium hydrogenphosphate hexahydrate (MP=5° C.), a mixture of sodium sulphate decahydrate+ammonium chloride+ammonium bromide (6-10 ° C.), a mixture of calcium chloride hexahydrate+magnesium bromide hexahydrate (MgBr₂·6H₂O)+strontium hydroxide (Sr(OH)₂) (MP=12° C.), an eutectic mixture of magnesium sulphate heptahydrate (MgSO₄·7H₂O)+water (MP=5° C.), an eutectic mixture of potassium chloride with water (MP=10.7° C.) and an eutectic mixture of ammonium chloride with water (MP=15.8° C.)

The type and the choice of the PCM material may be dependent upon liquid-solid phase transition temperature of the material and the heating or cooling desired for a particular application. In many applications, the PCM is selected so that it has a liquid-solid phase transition temperature of from 50° C. or less, more particularly from 30° C. or 25° C. or less. In still others, the PCM may be selected so that it has a liquid-solid phase transition temperature of from 10° C., 9° C., 8° C., 7° C., 6° C., 5° C., 4° C., 3° C., 2° C., 1° C. or less.

It should be noted in the description, if a numerical value or range is presented, each numerical value should be read once as modified by the term “about” (unless already so modified), and then read again as not so modified unless otherwise indicated in context. Also, in the description, it should be understood that an amount range listed or described as being useful, suitable, or the like, is intended that any and every value within the range, including the end points, is to be considered as having been stated. For example, “a range of from 1 to 10” is to be read as indicating each and every possible number along the continuum between about 1 and about 10. Thus, even if specific points within the range, or even no point within the range, are explicitly identified or refer to, it is to be understood that the inventors appreciate and understands that any and all points within the range are to be considered to have been specified, and that inventors possesses the entire range and all points within the range.

The PCM materials may be selected from those that can absorb from 100 kJ/kg to 250 kJ/kg of latent heat, more particularly from 150 kJ/kg to 240 kJ/kg of latent heat. PCMs typically have a sharp melting point and a high latent heat of absorption. Other characteristics of PCM materials include good cycling stability wherein the materials can be used many times for storage and release of heat as required by an application. The PCMs may further have little or no super cooling, no phase separation (especially in case of mixtures), good thermal conductivity, low vapor pressure, small volume change, extended chemical stability, and compatibility with the packaging material. The PCMs may further be non-toxic and non-flammable.

As discussed previously, the PCM materials usually cannot be used non-contained or in their neat form so they must typically be contained to avoid any negative consequences that would result if they are not contained. Beyond the obvious issues resulting from liquefied PCM escaping or spilling into the surrounding area from the thermal energy storage system or device, the PCM may be negatively affected itself if not contained in a moisture or gas impervious container or shell. For example, hygroscopic metal hydrates can absorb moisture from the environment if not contained in a moisture-proof container or shell, such that they lose their effectiveness. The absorption and dissolution of atmospheric gases into the PCM can also be detrimental. PCMs of long alkyl chain compounds, for example, can undergo oxidation if exposed to air so that they can degrade or lose their effectiveness as a PCM material for a particular application.

Accordingly, as discussed for the various embodiments described herein, encapsulating or containment methods can prevent both the escape of the PCM as well as protect the PCM from the outside environment. The container, shell or encapsulating material used to contain the PCMs is that having sufficient mechanical strength and imperviousness that can be maintained over time under the conditions in which the PCM is used. This includes maintaining its integrity under the temperatures and packing loads encountered, as well as withstanding the multiple heating and cooling cycles required for the PCM. Furthermore, the container or encapsulating material must be compatible with the PCM itself.

One particular material that can be used as an encapsulating or containment material to achieve this is high density polyethylene (HDPE). HDPE is contrasted with low-density polyethylene (LDPE) and linear low-density polyethylene (LLDPE), which are often severely affected when used with many PCMs. The density of non-crosslinked HDPE typically ranges from 0.93 g/cm³ to 0.97 g/cm³. HDPE typically has less branching than LDPE, which gives it higher tensile strength. HDPE may also include copolymers of ethylene and higher olefins such as 1-hexene or 1-butene. HDPE materials may have a melt flow index (MFI) of 0.2-4 g/10 min at 190° C. and 2.16 Kg or 0.2-0.6 g/10 min at 190° C. and 5 Kg or 1-10 g/10 min at 190° C. and 21.6 Kg, a tensile modulus of 900-1200 MPa, a tensile strain at break of 200-1000%, flexural modulus of 600-1400 MPa, a notched izod impact of 10-50 KJ/m² @ 23° C., heat deflection temperature (HDT) of 75-85° C. (at 0.45 MPa), Vicat softening temperature of 120-130° C. (at 10 N), melting point (as inferred from differential scanning calorimetry (DSC)) of 130-135° C., and thermal conductivity of 0.2 to 0.5 W/m·K). The HDPE material may be a non-crosslinked or crosslinked HDPE. In certain applications or embodiments, a crosslinked HDPE is used. Crosslinking increases the mechanical strength and thermal stability of the HDPE. Indeed, the environmental stress crack resistance (ESCR) and molecular weight of a crosslinked HDPE can be multiple times greater than that of the same non-crosslinked HDPE.

The crosslinked HDPE may be crosslinked or cured during or after being formed or configured into the encapsulating material body or shell used to contain the PCM. Crosslinking of polyethylene can be achieved by peroxide, electron beam and silane technologies. Both peroxide and electron beam crosslinking technologies are similar, as in both cases the generated radicals leads to the formation of a chemical bond between polymer chains. While peroxide technology necessitates a tight operating temperature window during the processing, electron beam technology is typically associated with high investment and processing costs. In comparison, silane cross-linking is proven most economical way to cross-link polyethylene based pipes and cables. Silane cross-linking technology may be a 2-step process, in which a vinyl silane is grafted onto polyethylene chains in the presence of a small amount of peroxide in an extruder in the first step. This is followed by the cross-linking of silane outside of the extruder, by exposing the extruded and/or formed HDPE in hot water. The cross-linking of silane can be accelerated with heat, moisture and catalyst.

With silane crosslinking, the cross-linked bonds are formed through silanol condensation between two grafted vinyl silane units, which connect the polyethylene chains with C—C—Si—O—Si—C—C bridges. The silane crosslinking agent may have the formula (X)₃Si—Y, where X is a hydrolyzable group, such as ethoxy, methoxy, or acetoxy, which reacts with water to form silanol (Si—OH), and Y is a functional organic group, such as vinyl, amino, epoxy, etc., both with and without alkylene (e.g., —CH₂—, —(CH₂)₃—) spacer. Examples of commercial silane crosslinking processes are the two-step process described in U.S. Pat. No. 3,646,155 (Sioplas® process) and the one-step process described in U.S. Pat. No. 4,117,195 (Monoplas® process), each of these patents being incorporated by reference in their entireties for all purposes. The Sioplas® process involves the peroxide activated grafting of the vinyl silane onto polyethylene in a compounding unit, such as co-kneader or twin-screw extruder, and the subsequent shaping of the grafted polymer in a conventional extruder with the incorporation of a catalyst master-batch. The Monosil® process combines grafting of vinyl silane onto the polymer, the addition of the catalyst and shaping of the grafted polymer in one-step. Alternatively, the silane cross-linkable polyethylene can be obtained via copolymerizing ethylene and vinyltrimethoxy silane. Other crosslinking methods using vinyl silane may also be used. Examples of other crosslinking methods are those described in Morshedian, J., et al., Polyethylene Cross-linking by Two Step Silane Method: A Review, Iranian Polymer Journal 18 (2), February 2009, pp. 103-128, which is incorporated herein by reference in its entirety for all purposes. Commercial examples of suitable silane crosslinking agents are those marketed as DYNASYLAN® functional silane products, available from Evonik Industries. Typical amount of vinyltrimethoxy silane grafted onto the polyethylene varies from 0.5 wt. % to 10 wt. % and still more particularly, from 2 wt. % to 7.5 wt. %.

Where crosslinked HDPE is used as the encapsulating material, the degree of HDPE crosslinking, as inferred by gel content, may range from 1% or more, more particularly from 10% or more crosslinking, and still more particularly from 20%, 30%, 40%, 50%, 60% or more crosslinking. In certain embodiments, the crosslinked HDPE may have from 90% or less crosslinking. In particular instances the HDPE may have from 10%, 20%, 30%, 40%, 50%, or 60% to 70%, 80%, or 90% crosslinking, with from 50% to 90% being particular useful. The amount of crosslinking agent used may be that that provides the desired degree of final crosslinking.

While HDPE, both non-crosslinked and crosslinked, may be used as an encapsulating or shell material for PCMs, certain mechanical properties of the HDPE can be affected when placed in contact with PCMs, particularly when such contact is maintained over long periods of time and under the conditions that the PCMs are used in thermal energy storage system applications. In particular, properties such as Young's modulus, yield stress, and ultimate tensile strength may all be affected.

One way to evaluate suitable HDPE materials with PCMs in thermal energy storage systems is by evaluating the HDPE's ESCR. This may include evaluating the HDPE encapsulating material using environmental stress crack resistance testing methods. For those HDPE materials used in the thermal energy storage systems described herein, a set of five injection molded tensile bars specimens of the HDPE material are immersed in the liquid PCM material at room temperature or temperatures that may range from 5° C. to 27° C. The samples are withdrawn at different intervals of time and evaluated for their tensile properties and for dimensional/weight changes, if any. While no or only a slight change (1-10% deviation from initial baseline values) in tensile properties and/or dimension during the stipulated testing period (e.g., up to 100 hr) is considered to indicate that the tested HDPE grade is suitable to use as an encapsulating material. A significant change in tensile properties and/or dimension (i.e., more than 10-20% deviation from initial baseline values) during the selected testing period (e.g., up to 100 hr) is considered to indicate that the tested HDPE grade is not suitable to use as an encapsulating material. The ESCR is the number of hours or the time it takes for this failure to be observed. In parallel, the chemical composition (by Fourier transform infrared (FTIR) and melting characteristics (DSC) of aliquots of PCM during the stipulated testing period are also evaluated. This is done in order to check if the PCM has undergone changes, if any, while in contact with encapsulating material over a prolonged duration.

Those HDPE materials that have an ESCR of 50 hours, 100 hours, 200 hours, 500 hours, 600 hours, 700 hours, 800 hours, 900 hours 1000 hours or more under the selected testing conditions are those that may be used as an encapsulating material.

Those HDPE materials, non-crosslinked or crosslinked, evaluated and that exhibit high ESCR may be used in conjunction with the PCM to form the thermal energy storage system or devices. This is accomplished by surrounding the phase change material with an encapsulating material or encapsulating shell so that the phase change material is fully contained therein to prevent passage of the phase change material out of the encapsulating material and/or to prevent direct contact of the phase change material with external objects, as well being moisture and gas impervious.

Typically, the HDPE material is melt processed with several processing additives. These may include such things as flow modifiers and functional additives, such as heat stabilizers, anti-oxidants, thermally conductive additives, nucleating agents, acid scavengers, mold release additives, ultraviolet (UV) light stabilizers, clarifiers, lubricants, slip additives, etc. Flow modifiers may include, for example, hydrocarbon fluids and silicone fluids. Antioxidants may include the commercial product IRGANOX 1010. Heat stabilizers may include the commercial product IRGAFOS 168. Light stabilizers may include the commercial product TINUVIN and hindered amine light stabilizers (HALS). Non-limiting examples of thermally conductive additives may include aluminum oxide (Al₂O₃), zinc oxide (ZnO), magnesia (MgO), aluminosilicates, graphene, and boron nitride. Nucleating agents may include calcium carbonate, titanium oxide, silicon dioxide, talc, nanoclay. Slip additives may include erucamide, behanamide, and oleamide.

Various encapsulating techniques may be used to encapsulate the PCM material in the HDPE. These may include injection molding, gas-assisted injection molding, water-assisted injection molding, thermo-forming, blow molding, blown film extrusion, and 3D-printing. In certain instances, the HDPE encapsulating shell may be formed around an existing PCM material while it is in a frozen or solid state. In other embodiments, the HDPE material is formed into the encapsulating body of a desired shape and liquefied PCM is introduced into an opening of the HDPE encapsulating body and sealed therein. Sealing methods may include microwave sealing, photothermal sealing, hot-press sealing, etc. The final HDPE shell or encapsulating body may be flexible or rigid.

The amount of PCM material within the HDPE encapsulating body may be less than the total volume of the HDPE encapsulating shell or body to accommodate volume changes of the PCM due to the transition between the liquid and solid phases. The amount of PCM material contained in the thermal energy storage system may range from 10% to 99% of the total volume of the thermal energy storage system, more particularly from 10%, 20%, 30%, 40%, or 50% to 60%, 70%, 80%, or 90% of the total volume of the thermal energy system. In thermal energy storage system, the PCM is contained within walls of the HDPE enclosure wherein, for example, from 85% to 90% by volume of the HDEP enclosure is filled with the PCM. This accommodates the change of volume when the PCM changes between the liquid-solid phase, which, for example, may be from 7% to 10%.

The HDPE encapsulating body and the thermal energy storage system or device may be formed into a variety of shapes. These may be spherical and non-spherical, cylindrical or non-cylindrical, linear or non-linear. In certain embodiments, the thermal energy storage system and/or the HDPE encapsulating body may be formed into a small spheres or bodies having a diameter or particle size of from 10 mm to 40 mm. In other embodiments, the thermal energy storage system and/or the HDPE encapsulating body may be formed into an elongated body or straw, which may be cylindrical or non-cylindrical, having a diameter or transverse width of from 2 to 5 mm and a length of from 10 mm to 1000 mm. The wall thickness of the encapsulating material or body may be from 3 mm, 2.5 mm, 2 mm, 1 mm, 0.05 mm or less.

Where the thermal energy storage system or devices are small in size, a plurality of them may be contained within a larger container, such as a flexible bag or a panel or other device, to contain them during use or storage.

The thermal energy storage systems or devices may be used in a variety of different applications. They can be used to replace or supplement cooling or refrigeration equipment, such as used in storage, shipping, etc. They can be placed in direct contact with items that need to be cooled or maintained at desired temperatures or they may be placed in locations nearby or in rooms or areas where items needing cooling are located. The thermal energy storage systems can be used to cool or maintain the temperature of food and drinks, medicine or medical supplies, electronics and batteries, blood or bio-samples, etc. In certain applications, refrigeration equipment can be used to freeze the PCM of the thermal energy storage device using off-peak electricity so that during peak electricity use, the thermal energy storage system can be used in areas where cooling or refrigeration is needed without using electrical refrigeration equipment and thereby reducing electricity demand during the peak usage times

The following examples serve to further illustrate various embodiments and applications.

EXAMPLES Example 1

Environment Stress Cracking Resistance (ESCR) describes the accelerated failure of polymeric materials, as a combined effect of environment, temperature and stress. The failure mainly depends on the characteristics of the polymeric material, the chemical to which the polymeric material is exposed, the exposure condition and the magnitude of the stress. The chemical can be in the form of liquid, semi liquid, paste or gas. Though there are different test methods available for assessing the resistance of polymeric materials to various chemicals, the most commonly employed method of inferring the ESCR characteristics of the plastic parts is the constant strain method (ASTM D543/ISO 22088-3 standard).

According to the constant strain method, one set of injection molded tensile bars of HDPE (typically 5 numbers) were exposed or immersed to chemical environment (in the present case, it is 100% of different PCMs) for a fixed time under a known strain (in the present case, it is 0% strain). Another set of injection molded tensile bars of HDPE (control samples, typically 5 numbers) were exposed to the same environmental conditions (for instance, same temperature; in the present case it is 23° C.), but without exposing to the chemical. The exposed samples were visually examined and mechanical properties were tested. The mechanical property results of exposed HDPE samples were compared with those of control samples, to infer the ESCR characteristics.

Seven different grades of HDPE were tested. These include HDPE-1 (blow molding grade for industrial containers), HDPE-2 (broader molecular weight, blow molding of large parts such as fuel tanks), HDPE-3 (blow molding of Jerry cans), HDPE-4 (high molecular weight, containers for bleach, detergents, industrial chemicals), HDPE-5 (unimodal, injection and compression molding grade) HDPE-6 (bimodal pipe grade), and HDPE-7 (bimodal pipe grade for high pressure). The samples vary in terms of melt flow index (MFI), type of comonomer, density, modality (unimodal, bimodal and multimodal). These were each exposed to four different organic phase change materials (tetradecane (PCM-1), polyethylene glycol (PCM-2), 1-decanol (PCM-3) and methyl laurate (PCM-4)) for different time period (3, 15, 50 and 100 hours) at room temperature (23° C.) in order to infer their ESCR characteristics.

The mechanical properties (tensile strength (MPa) and nominal strain at break (%)) of the tensile bars were measured after the completion of the specified exposure period and compared with those of control samples. The tensile properties for this experiment were determined using an Instron testing machine, with a test speed of 50 mm/min in accordance with ISO 527.

The percentage (%) retention of tensile strength/nominal strain at break is calculated as per the following Equations (1) and (2):

$\begin{matrix} {{{Retention}\mspace{14mu} {of}\mspace{14mu} {Tensile}\mspace{14mu} {Strength}\mspace{14mu} (\%)} = {100 \times \left( \frac{{Tensile}\mspace{14mu} {Strength}\mspace{14mu} {After}\mspace{14mu} {the}\mspace{14mu} {Exposure}\mspace{14mu} {to}\mspace{14mu} {PCM}}{{Tensile}\mspace{14mu} {Strength}\mspace{14mu} {Without}\mspace{14mu} {Exposure}\mspace{14mu} {to}\mspace{14mu} {PCM}} \right)}} & (1) \\ {{{Retention}\mspace{14mu} {of}\mspace{14mu} {Nominal}\mspace{14mu} {Strain}\mspace{14mu} {at}\mspace{14mu} {Break}\mspace{14mu} (\%)} = {100 \times \left( \frac{{Nominal}\mspace{14mu} {Strain}{\mspace{11mu} \;}{After}{\mspace{11mu} \;}{the}\mspace{14mu} {Exposure}\mspace{14mu} {to}\mspace{14mu} {PCM}}{{Nominal}\mspace{14mu} {Strain}{\mspace{11mu} \;}{Without}\mspace{14mu} {Exposure}\mspace{14mu} {to}\mspace{14mu} {PCM}} \right)}} & (2) \end{matrix}$

The performance of a particular grade of HDPE when exposed to a particular PCM is ranked with the criteria, depicted in the Table 1 below. While <65% lowering of nominal strain is indicative of embrittlement of HDPE, greater than a 140% increase of nominal strain can be attributed to the plasticization.

TABLE 1 % Retention Ranking of Tensile strength % Retention of Nominal Strain H >90 80-130 M 80-89 65-79; 130-140 L <79 <65 or >140

The ESCR performance of different grades of HDPE, in terms of the retention of tensile strength and the retention of nominal strain when exposed to different organic PCMs are presented in the Table 2 and Table 3, respectively.

TABLE 2 Type of HDPE PCM-1 PCM-2 PCM-3 PCM-4 HDPE-1 M H H M HDPE-2 H H H H HDPE-3 M H H H HDPE-4 H H H H HDPE-5 H H L H HDPE-6 M H H M HDPE-7 M H H H

TABLE 3 Type of HDPE PCM-1 PCM-2 PCM-3 PCM-4 HDPE-1 H H H H HDPE-2 L H H H HDPE-3 L H H H HDPE-4 H M M H HDPE-5 L M L L HDPE-6 H H H H HDPE-7 H H H H

As shown in the Tables 2 and 3, the retention of tensile strength and the nominal strain at break were found to vary when different HDPEs were exposed to different PCMs. Analysis of the data depicted in the Tables 2 and 3, the following increasing trend of ESCR performance of HDPE grades can be ascertained as follows:

HDPE-5<HDPE-2<HDPE-3<HDPE-4<HDPE-1<HDPE-6<HDPE7

A similar trend is observed when different HDPE grades are exposed to the commercially available PCMs, such as SP5 (inorganic PCM) and RT5 (organic PCM), supplied by Rubitherm GmbH.

While the invention has been shown in some of its forms, it should be apparent to those skilled in the art that it is not so limited, but is susceptible to various changes and modifications without departing from the scope of the invention. Accordingly, it is appropriate that the appended claims be construed broadly and in a manner consistent with the scope of the invention. 

We claim:
 1. A thermal energy storage system comprising: a phase change material; and an encapsulating material surrounding the phase change material so that the phase change material is fully contained therein to prevent passage of the phase change material out of the encapsulating material and/or prevent direct contact of the phase change material with external objects, the encapsulating material being a high density polyethylene (HDPE).
 2. The thermal storage system of claim 1, wherein: the HDPE encapsulating material has an environmental stress crack resistance (ESCR) while in contact with phase change materials at 5° C. to 27° C. of from 100 hours or more.
 3. The thermal energy storage system of claim 1, wherein: the phase change material has a liquid-solid phase transition temperature of from 50° C. or less at standard pressure.
 4. The thermal energy storage system of claim 1, wherein: the phase change material has a liquid-solid phase transition temperature of from 10° C. or less at standard pressure.
 5. The thermal storage system of claim 1, wherein: the phase change material is a selected from at least one of an alkane, an alcohol, an organic acid, an ester, polyethylene glycol, an inorganic salt hydrate, a mixture of inorganic salts and/or inorganic hydrates, eutectic mixtures of organic-organic materials, eutectic mixtures of organic-inorganic materials, and eutectic mixtures of inorganic-inorganic materials.
 6. The thermal storage system of claim 1, wherein: the HDPE material is a cross-linked HDPE.
 7. The thermal storage system of claim 6, wherein: the HDPE material has from 90% or less crosslinking.
 8. The thermal storage system of claim 1, wherein: the encapsulating material has a wall thickness of from 3 mm or less.
 9. The thermal storage system of claim 1, wherein: encapsulating material contains a heat conducting additive.
 10. The thermal storage system of claim 1, wherein: the thermal storage system is formed in at least one of 1) a sphere having a diameter of from 10 mm to 40 mm, and 2) an elongated cylindrical body having a diameter of from 2 to 5 mm.
 11. A method of forming a thermal energy storage system comprising: surrounding a phase change material with an encapsulating material so that the phase change material is fully contained therein to prevent passage of the phase change material out of the encapsulating material and/or to prevent direct contact of the phase change material with external objects, the encapsulating material being a high density polyethylene (HDPE).
 12. The method of claim 11, wherein: the HDPE encapsulating material has an environmental stress crack resistance (ESCR) while in contact with phase change materials at from 5° C. to 27° C. of from 100 hours or more.
 13. The method of claim 11, wherein: the phase change material has a liquid-solid phase transition temperature of from 50° C. or less at standard pressure.
 14. The method of claim 11, wherein: the phase change material has a liquid-solid phase transition temperature of from 10° C. or less at standard pressure.
 15. The method of claim 11, wherein: the phase change material is a selected from at least one of an alkane, an alcohol, an organic acid, an ester, polyethylene glycol, an inorganic salt hydrate, a mixture of inorganic salts and/or inorganic hydrates, eutectic mixtures of organic-organic materials, eutectic mixtures of organic-inorganic materials, and eutectic mixtures of inorganic-inorganic materials.
 16. The method of claim 11, wherein: the HDPE material is a cross-linked HDPE.
 17. The method of claim 16, wherein: the HDPE material has from 90% or less crosslinking.
 18. The method of claim 11, wherein: the thermal storage system is formed in at least one of 1) a sphere having a diameter of from 10 mm to 40 mm, and 2) an elongated cylindrical body having a diameter of from 2 to 5 mm; and wherein the encapsulating material has a wall thickness of from 3 mm or less;
 19. The method of claim 11, wherein: encapsulating material contains a heat conducting additive.
 20. The method of claim 11, wherein: the encapsulating material is formed into a selected shape by at least one of injection molding, gas-assisted injection molding, water-assisted injection molding, thermo-forming, blow molding, blown film extrusion, and 3D-printing. 